With the sharp increase of vehicles on roads in the recent years, driving has not stopped from being more challenging and dangerous. Roads are saturated, safety distance and reasonable speeds are hardly respected, and drivers often lack enough attention. Without a clear signal of improvement in the near future, leading car manufacturers decided to jointly work with national government agencies to develop solutions aimed at helping drivers on the roads by anticipating hazardous events or avoiding bad traffic areas. One of the outcomes has been a new type of wireless access called Wireless Access for Vehicular Environment (WAVE) dedicated to vehicle-to-vehicle and vehicle-to-roadside communications. While the major objective has clearly been to improve the overall safety of vehicular traffic, promising traffic management solutions and on-board entertainment applications are also expected in this field.
When equipped with WAVE communication devices, cars and roadside units form a highly dynamic network called a Vehicular Ad Hoc Network (VANET), a special kind of Mobile Ad Hoc Network (MANET) where vehicles communicate with one another through wireless infrastructures to the Internet using a multihop-to-infrastructure routing protocol.
Although designed for Low power and Lossy Networks (LLNs), such as sensor networks, the Routing Protocol for LLNs (RPL) is clearly an ideal candidate for routing in a VANET. For instance, RPL is a distance vector routing protocol that builds a Destination Oriented Directed Acyclic Graph (DODAG, or simply DAG) in addition to a set of features to bound the control traffic, support local repair, etc., and provides a flexible method by which each node performs DODAG discovery, construction, and maintenance. However, RPL has been inherently designed for fixed low-speed networks interconnecting highly constrained devices (in terms of CPU processing, memory, and energy). Thus, RPL does not use keepalive mechanisms to maintain routing adjacencies, but rather relies on dataplane verification to detect that a next-hop is alive when sending data packets. Although RPL could be augmented with keepalives, this does not allow for efficient connectivity maintenance in highly mobile (fast-moving) networks such as a VANET.